An ion trap with four segmented blade electrodes used to trap a linear chain of atomic ions for quantum information processing

Emily Edwards

Quantum computing has hit another "milestone" with US researchers today unveiling the development of a small quantum computer that can be reprogrammed.

How ion traps work

Key points

Quantum computer developed with five qubits

Computer can solve three algorithms in a single step using quantum effects

Experts say technology is a milestone but significant challenges remain

Many research groups have previously created small, functional quantum computers, but most of these have been only able to solve a single problem.

However in today's Nature journal, Shantanu Debnath and colleagues at the University of Maryland reveal their new device can solve three algorithms using quantum effects to perform calculations in a single step, where a normal computer would require several operations.

Although the new device consists of just five bits of quantum information (qubits), the team said it had the potential to be scaled up to a larger computer.

In traditional computing bits are either 1 or 0, while in a quantum computer, qubits can be both numbers at the same time.

This has the potential to provide faster computation in areas such as materials sciences, searching large databases and data security and encryption

In this latest device, the computer's qubits are individual ions — charged atoms — that are trapped in a line using magnetic fields and then manipulated using lasers.

"By directly connecting any pair of qubits, we can reconfigure the system to implement any algorithm," lead author Mr Debnath said.

"While it's just five qubits, we know how to apply the same technique to much larger collections."

The team believes that eventually more qubits— perhaps as many as 100 — could be added to their quantum computer module.

In an accompanying opinion piece in Nature, Sydney University Professor Stephen Bartlett said the development was a milestone that the community had been working toward.

"One of the big achievements is to get all five trapped ions working together. Many groups have done five or even more, but the advance [here] is that with the five qubits, they could do anything they wanted," Professor Bartlett, from the Centre for Engineered Quantum Systems.

"This is going to show this approach has some legs to it and is going to work in the long run," he said.

Laser pulses key to system

Mr Debnath said the key to the new device was a system of laser pulses that drove the quantum logic gates, which operate like the switches and transistors that power ordinary computers.

The lasers push on the ions allowing any two ions in the module to interact via their strong electrical repulsion.

The ability to reconfigure the laser beams to control specific ions in the line was a key advantage, said Mr Debnath

"By reducing an algorithm into a series of laser pulses that push on the appropriate ions, we can reconfigure the wiring between these qubits from the outside," he said in a statement.

"It becomes a software problem, and no other quantum computing architecture has this flexibility."

Professor Lloyd Hollenberg, at the University of Melbourne, said while the development was "a significant step" there were still many challenges ahead for the technology.

"I think it is well known these initial demonstrations are fine in one dimension, but if you want to scale up to a universal computer that is error-corrected, you would have to go two-dimensional," said Professor Hollenberg, Centre of Excellence for Quantum Computation and Communication Technology deputy director.

One of the advantages of using ions was that they are naturally occurring, he said.

However, Professor Hollenberg noted other quantum technologies such as those using superconductor materials had also reached a similar level of computing, while atomic spins in silicon with similar programmable flexibility had great potential for scaling up in 2D, and qubits based on photons would interface naturally with communication systems.

"The work of Debnath [and colleagues] is a significant proof of principle for quantum technology based on atomic qubits," Professor Hollenberg said.

"It shows with such qubits you can reach this level of complexity in terms of quantum circuits, and on the same hardware, they are able to implement different quantum gates on different circuits."

Error rate needs to be addressed

But both Professors Hollenberg and Bartlett highlighted the new device still had a relatively high error rate of 2 per cent that would need to be addressed.

"The error rate isn't good enough," Professor Bartlett said. "It's good enough for the demonstration, but they would want to get the error rate down below 0.1 per cent — that is the number that most quantum architects think of as the largest error rate a quantum computer can tolerate."

Professor Bartlett said the development showed the field was moving inexorably forward.

"We are on the cusp of something," he said. "In the next five or 10 years this is going to go out of the physics labs and start to make a difference in terms of technology.

"We are probably not going to have quantum computers on our desk, but in that timescale we are going to start to see quantum computing technology in the way we do large-scale computing and cloud computing."